Electric cable



April 26, 1949. M DDDD CK 2,468,091

5. I \lifi Patented Apr. 26, 1949 ELECTRIC CABLE Bruce H. Maddock,Fanwood, N. J., assignor to Federal Telephone and Radio Corporation, NewYork,'N. Y., a corporation of Delaware Application January 18, 1945,Serial No. 573,350

1 Claim. (01. 17424) This invention relates to an electric cable in Ywhich a large part of the surrounding dielectric,

particularly that in close proximity to the conductor, is formed of air,the conductor being spaced from its surroundings by solid material whichoccupies only an extremely small proportion'of the intervening space.

It has been recognized already for several decades that it is essentialto design electric cables with high insulating qualities for theconductors and with a low inductive capacity. It was also well knownthat the provision of air spaces around the conductor served bothpurposes, namely, to'provide extra insulation for and to reduce thecapacity of the cable. In order to achieve the desired results, it hasbeen proposed to surround the conductor with a covering containinglongitudinal or radial air spaces. Thus, it can be seen that there was aclear tendency in the construction of transmission cables utilized inconnection with conductors, microphones, television and radio frequencydistribution to lower the capacity and, therefore, to enlarge the airspaces in the covering of the cables and to provide predeterminedportions of the interior of the cable with a lower average dielectricconstant. This tendency brought about a cable having a cross-sectionshaped to provide-a plurality of arms. Whereas these previousconstructions proved to have a great proportion of solid material inclose proximity to the conductor, and in some cases surrounded theconductor even entirely with solid material, with the result that thedesired low capacity of the cable could not be achieved, the laterdevelopment in the art went into the direction of providing a large partof the dielectric surrounding the conductor, particularly in closeproximity to the latter, of air. The conductor was in these latterconstructions spaced from its surrounding by solid material whichoccupied only a small proportion of the intervening space, whereby thespacing was effected by a cord or string of insulating material woundover the conductor as an open helix. Although this later developmentappeared to be an improvement over the older constructions, it stilllacked entirely satisfactory results, apparently due to the limitedradial extension of the air space around the conductor.

The present invention overcomes all drawbacks of these known cableconstructions and provides a cable of lowest capacity. Whereas theoldest constructions for electric cables provided air spaces in more orless greater distances from the cable and the space in close proximityto the conductor was occupied in a large proportion by solid material,thus lacking sufficient air space in close proximity to the conductor,the later constructions provided a larger proportion of air space inclose proximity to the conductor only, which air space had far toolimited radial extension. Both kinds of constructions did not provide,therefore, entirely satisfactory results.

The present invention is based on a newly found principle, according towhich suflicient air space in close proximity to the conductor and alsosuificient radial extension of that air space are essential in order toachieve the desired results for high frequency cables.

It is, therefore, the object of the present invention to provide anelectric cable which is constructed for particularly low capacity to beused substantially for high frequency purposes.

It is another object of the present invention to provide an electriccable in which the amount of solid dielectric surrounding the cable issubstantially reduced.

It is still another object of the present invention to provide anelectric cable which possesses a very low average dielectric constantparticularly in close proximity of the conductor.

It is a further object of the present invention to provide an electriccable in which the condoctor is spaced from its surroundings by solidmaterial which occupies only a small proportion of the interveningspace, thus the large proportion of the intervening space being occupiedby air which air gap is extended radially into the surrounding solidmaterial.

With these and other objects in view the present invention will becomeapparent in the following specification and drawing, in which:

Figure 1 is a cross section of an electric cable; and

Fig. 2 is a longitudinal section along the lines 2-2 of Fig. 1.

As shown in the drawing, the cable comprises a central wire conductor laround which is wound insulating spacing 2. The latter has preferablytheform of an open helix of relatively long pitch, through any otherform can be used for the spacing means, and consists of any convenientinsulating plastic material as preferably of the lowloss type such aspolyethylene, polystyrene, or compounds which may include these, orequivalent materials. Around the spacing member 2 is disposed acylindrical jacket 3 having inwardly directed radial webs 4 each ofwhich engages the insulating helix 2 merely at a few points which aredistanced by the predetermined pitch of the helix 2, whereas theremaining portions of the inner ends of the webs I are spaced by an airgap 5 from the conductor I. The helix 2 engages any point of theconductor I only on a part of its periphery. The cylindrical jacket 3and the webs l are preferably of the same insulating plastic material asdescribed before for the helix 2. Whereas four webs are shown, it isobvious that a fewer or greater number can be employed.

The construction according to the present invention substantiallyincreases the radial length of the average air gap around the conductor,which air gap comprises two connected air zones, namely, one inner zoneachieved by the spacing means and an outer zone which is achieved by aplurality of air spaces formed by the cylindrical jacket 3 and each pairof adjacent webs 4. This increase of the radial length of the averageair gap around the conductor is achieved without in creasing the portionof solid insulating material in close proximity to the conductor whileat the same time adequately mechanically supporting the centerconductor. Thus the provision of two interconnected air zones in theconstruction of the electric cable resulted in a particularly lowcapacity characteristic for the cable.

While I have disclosed the principles of my invention in connection withseveral difierent embodiments, it will be understood that theseembodiments are given by way of example only and not as limiting thescope of the invention as set .forth in the objects and the appendedclaim.

4 WhatIclaim is: In an electriccable of the type including a centralconductor, electrically insulating means surrounding said conductor thatcomprises a cord of low loss insulating material wound therearound inthe form of an open helix of relatively long pitch, and a tubular memberof low loss insulating material surrounding said central conductor andthe cord wound therearound but retained in spaced relationship theretoby longitudinally extending ribs, formed integrally with and extendingradially inwardly from the interior wall of said tubular member, saidribs tapering inwardly and pressing against the helically wound cord butnot against the central conductor.

BRUCE H. MADDOCK.

REFERENCES crrEn The following references are of record in the file ofthis patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Great Britain, 1911Number Number

